CIRCUIT BOARD IONIC CLEANLINESS MEASUREMENT --
WHAT DOES IT TELL US?
by
Dr. Jack Brous
Alpha Metals, Inc.
Jersey City, New Jersey
1. Introduction
In the early 1970's the only fluxes permissible for electronics
In the early 1970's the only fluxes permissible for electronics
manufacturing for military applications were types R (pure rosin
only) and RMA (rosin based, mildly activated). Before any of
these fluxes could be approved and established on the military
"qualified products list", it was necessary to demonstrate that
B
they were non-corrosive and did not contain ionic halides in
excess of established critical levels. All fluxes were required
to pass a silver chromate paper test which sensitively indicates
the presence of halide ions. Additionally, the flux was subjected
to a copper mirror corrosion test which is a sensitive indicator
of any corrosive behavior of the flux toward copper.
Because of the desirability for using stronger, more active fluxes
in electronic assembly, the military agencies decided to include
the category RA (fully activated rosin) on their approved flux
list. RA fluxes do not, as a rule, pass the copper mirror or
chromate paper tests and there was, therefore, increased concern
about allowing the more highly active rosin flux into military
electronics manufacture.
Although circuit assemblies are ordinarily cleaned after assembly
soldering, traces of RA fluxes, left after the final cleaning,
would be considered a greater risk than those of RMA or R types.
It was, therefore, imperative to thoroughly clean these assemblies
to provide assurance that the potentially harmful active residues
had been thoroughly removed.
2. Cleanliness Measurement
The RA fluxes most commonly use halide salts, such as amine hydro-
chlorides, as additives to enhance fluxing activity. In cleaning,
these additives are removed along with the rosin residue. If,
however, some of the residue is not removed in the cleaning
process, some of the halide activators can remain behind, en-
trapped in the solid rosin residue.
Such highly ionic material can be very sensitively detected and
measured if brought into a solution containing water. Using
electrochemical conductometric methods, ionic materials, which are
easily detectable, can act as tracers for the presence of any RA
flux residue remaining after the cleaning process.
It is well known, to physical and electrochemists, that the addi-
tion of a strongly ionized salt to deionized water will enhance
its electrical conductance to a degree that is nearly proportional
to the concentration of the salt {1,2}. Conductance measurement
can thus be used to indicate concentration of an ionic salt ex-
tracted into a solution.
This was discussed, by T.F. Egan of Bell Laboratories, in a paper
published in 1973{3}, who used this method to measure ionic plating
salt residues on plated components. The method described by his
was, however, unsuitable to measure the ionic content of rosin flux
residues from an electronic assembly. Rosin is insoluble in pure
water, therefore, any ions present in the residue would be trapped
in a matrix of the solid, insoluble rosin and not brought into
aqueous solution where they could be measured.
In 1972, W. Hobson and R. DeNoon, at the Naval Avionics Center,
Indianapolis, IN, showed that rosin flux residues could be solu-
bilized in a solvent mixture containing 75% isopropanol and 25%
water{4}. This mixture could be used as the solvent base for the
conductometric measurements. The alcohol, at that level, is able
to solubilize any rosin traces and the water is needed to sustain
ionization for a conductivity measurement.
3. Testing Methods
Several testing procedures and instruments were established to
monitor the levels of residual ions extracted in alcohol/water
mixtures:
a. The original method of Egan was adapted by Hobson and DeNoon
to include the use of an isopropyl alcohol/water mixture as
the extracting solvent. In this method, the assembled board
is flushed with a pre-determined quantity of the mixture and
the resistivity of the "contaminated" solvent measured{4}.
b. An instrument was developed in which an assembly was immersed
in an agitated, fixed volume of a mixture of isopropanol/
water{5}. The resistivity of the solution is monitored
until there is no further indication of resistance change.
The effective level of ionic contamination can then be calcu-
lated from the change in solution resistivity. This instru-
ment, utilizing extraction into a static volume of alcohol/
water mixture was named the Omega Meter.
c. Another process was developed utilizing a continuously re-
circulating system in which the solvent mixture was deionized
by passing it through an ion-exchange column before recircu-
lating it back into the extraction tank{6,7}. During the
course of ionic extraction from an assembly, the conductivity
of the mixture falls continuously as the sample and solution
are cleaned. The integrated ionic measurement, over time, is
indicative of the total extracted ionic material.
This process, known as the dynamic extraction method, will
more sensitively indicate the completeness of the extraction.
This process is used in the Ionograph.
4. Cleanliness Standards - How Clean is Clean?
In 1977-78, a program was run at the Naval Avionics Center,
Indianapolis, to evaluate various ionic testing systems and to
attempt to establish ionic residue levels for ionic fluxes which
would be "safe" if left on circuit assemblies{8}. At a meeting
held in Indianapolis, February 1978, all of the data of the program
was reviewed and pass/fail limits were established for ionic clean-
liness levels of electronic assemblies for military applications.
These values were different for the different instruments which
were available since ionic residues were extracted from the assem-
blies with various degrees of efficiency. These efficiency, or
equivalence factors, were determined from the extensive data accum-
ulated in the prior testing program and resulted in recommendations
for pass/fail limits for general military requirements.
These values, subsequently, became generally adopted as the control
levels for the cleaning processes used to remove rosin flux resi-
dues{9} and were widely accepted for industrial non-military as
well as the military applications for which there were developed.
In time, they were also accepted by many as the cleanliness
standards for assemblies made with non-rosin fluxes, such as
water-soluble (OA type) and synthetic-activated (SA type) fluxes.
In short, these tests and their associated values, which were de-
rived for RA type rosin fluxes, were gradually and arbitrarily
extended to cover a variety of other flux types.
Other processing operations were included, as well, in this type
of testing to these same limits. One example is the ionic testing
of bare boards, either prior to or after application of solder
masks. While it is true that the presence of contaminants, on a
laminate surface, could affect subsequent coating adherence as well
as electrical characteristics, specific critical levels could
differ significantly from those established for activated rosin
fluxes.
Another example is in applying ionic testing to circuits which are
assembled, entirely or in part, using solder pastes. In such
precesses, the paste is applied in very localized areas. During
the reflow process the flux runs out in these limited areas and is
not distributed over the entire board as is the case for the wave
soldering process. Ionic measurement of these assemblies, after
cleaning, assumes that any residual ionic contamination is distri-
buted over the entire surface of the board since the total ex-
tracted ionic material is averaged over the estimated surface area
of the whole assembly. In using solder pastes, however, traces of
flux residues, which are present, will be concentrated in the very
much smaller areas of initial application and thus would represent
a much higher locallized contamination level that is indicated by a
general ionic extraction test.
5. How Clean is Good?
Perhaps a more pertinent question, that should be asked, is "How
clean should an assembly be to be functionally good?" An ultimate
test must be one of performance rather than the processing standard
of a measured ionic level. Unfortunately, ionic residue levels
have, in themselves, become confused in many minds as a criterion
of "goodness" or "badness" of the product. In particular, attempts
have been made to correlate ionic levels to more functional indica-
tors such as Surface Insulation Resistance (SIR) testing. SIR is
a measure of how various materials and processes affect the elec-
trical insulating characteristics of the laminate surfaces between
conductors.
In the earlier history of electronic assembly, when discreet wiring
was used to connect the various components, insulation between
conductors was derived from the insulating coating on the wires.
With the advent of printed wiring assemblies, the insulation became
the bare or solder-masked laminate spaces between the printed
wires. This exposed type of insulation is potentially more sus-
ceptible to the effects of residues from the soldering and cleaning
processes, particularly if the assembly is to be subjected to
various conditions of temperature and humidity. A measurement of
the electrical effects of the processing and chemicals used in the
assembly would, therefore, be most indicative of functional per-
formance in the subsequent application.
We know, today, that is not merely the presence or absence of ions,
as such, that determines good or bad behavior of a material resi-
due. Some of the poorest electrical behavior experienced has been
seen with surfaces which had been contaminated with polyglycols
and polyglycol surfactants{10,11}. These materials, although
completely non-ionic, may, nevertheless, have disastrous effects on
SIR and Electromigration if they are residual on the laminate sur-
face. Such materials have often been used in the formulation of
many water-soluble assembly fluxes and reflow and hot air solder-
levelling fluxes used in PCB fabrication.
To compound the problems caused by these materials, many of them
are absorbed into the surface of the plastic laminate when exposed
to the high temperature of the soldering or HASL or reflow pro-
cesses{11,12. They are then not easily removable in normal
aqueous or solvent cleaning processes. These retained materials
can give rise to serious degradation of the subsequent measured
values of SIR.
The most important factor determining a material's ability to
affect SIR is its ability to absorb water molecules from the am-
bient air. The hygroscopic nature of the molecule determines its
ability to interact with atmospheric water molecules to form multi-
layer films of water on the surface. Such water films are, of
themselves, slightly conductive and can result in degradation of
the SIR. The presence of ionic materials on the surface, which
could dissolve in this water film would serve only to exacerbate
the surface conductance and further lower SIR.
Although many polyglycols are examples of some of the worst of-
fenders in degrading SIR, such non-ionic materials are not the
only electrically hazardous materials. The key point of commonali-
ty of such materials is their ability to absorb moisture from the
ambient atmosphere (hygroscopicity). This property can be asso-
ciated with ionic or non-ionic materials, but is not universal for
either type.
6. "No Clean" Fluxes
In recent years, under the impetus of environmental protection, a
new breed of fluxes has evolved - "no clean" fluxes. These fluxes
are composed largely of weakly-ionized organic acids at low solids
levels which, at soldering temperatures, are sufficiently active to
provide good cleaning of the metallic surfaces and wetting by the
molten solder.
In most instances, much of the organic acid is volatillized at the
soldering temperatures or neutrallized by chemical reaction. Some
traces of the organic acids, however, are always present after the
soldering. Some of these have very low levels of hygroscopicity
and exhibit good electrical performance if they are limited to con-
trolled amounts. It has been shown, however, that excessive levels
of these organic acid residues can significantly degrade electrical
characteristics such as SIR and Electromigration{13,14}.
Conventional ionic extract testing could be of great value as a
quality control tool in a production environment. This test can be
used as a periodic check of the ability of the "no clean" process
to leave residue amounts in a consistent range below levels that
can seriously affect electrical characteristics. Significant in-
creases of ionic levels, in a periodic testing program, would then
indicate changes in the process which result in heavier residue
levels and their associated effects on the electrical character-
istics of the board surface.
It would be necessary, however, to initially develop the information
to define the "normal" ionic operating range for a specific
"no clean" process and flux. Once established, further periodic
ionic tests could serve as a rapid monitoring check - as compared
to lengthy and more difficult SIR tests - to indicate the degree of
control maintained over the "no clean" process and flux.
CONCLUSIONS
- Ionic extract testing is a valuable testing technique for
monitoring and controlling the effectiveness of a cleaning
process.
- Original pass/fail limits were established for wave soldering
processes using rosin fluxes. These limits are not necessarily
applicable to other types of fluxes and other soldering
processes.
- Ionic extract values are not necessarily valid indicators of
the "goodness" or "badness" of the final assemblies in terms
of the electrical properties of their laminate surfaces.
- Ionic measurement can be very valuable in controlling pro-
cesses using "no-clean" flux technology. This can be used in
monitoring amounts of flux solids applied as well as levels of
residues remaining on assemblies after soldering.
REFERENCES
1. J.C.Bockris and A.K.N.Reddy, Modern Electrochemistry, Plenum
Press, New York (1970)
2. F. Daniels and R.A.Alberty, Physical Chemistry, 2nd Ed.,
John Wiley & Sons, New York, London (1961)
3. T.F.Egan, Determination of Plating Salt Residues, Plating,
50(4), 350-354 (1973)
4. W.T.Hobson and R.J.DeNoon, Printed Wiring Assemblies,
Detection of Ionic Contaminants On, Materials Research
Report 3-72 (1972) Naval Avionics Center, Indianapolis, IN
5. E. Wolfgram, Means and Method for Measuring Levels of Ionic
Contamination, U.S.Patent 4,023,931 (May 17, 1977)
6. J. Brous, Self-Purging Apparatus for Determining the
Quantitative Presence of Derived Ions, U.S.Patent 3,973,572
(Aug. 10, 1976)
7. J.Brous, Evaluation of Post-Solder Flux Removal, Welding
Journal Research Supplement, 444s-444s (Dec. 1975)
8. W.Hobeen and R.J.Donoon, Review of Data Generated with
Instruments Used to Detect and Measure Ionic Contaminants
on Printed Wiring Assemblies, Materials Research Report 3-
78 (1978) Naval Avionics Center, Indianapolis, IN
9. MIL-P-28809, Military Specification, Printed Wiring
Assemblies
10. F.M.Zado, Proceedings of Technical Program-NEPCON, 1979
Philadelphia, PA, 3876-354
11. J.Brous, Water Soluble Flux and Its Effect on PC Board
Insulation Resistance, Electronic Packaging and Production,
July 1981, 79-87
12. J.Brous, Electrochemical Migration and Flux Residues -
Causes and Detection, Proceedings of the Technical Program,
NEPCON West 1992, 387-394
13. L.A.Guth, To Clean or Not to Clean?, Circuits
Manufacturing, Feb. 1989
14. J.E.Sohn and U.Ray, Weak Organic Acids and Surface
Insulation Resistance, IPC Technical Paper 1081, IPC,
Lincolnwood, IL June 1994
--
Karen Tellefsen
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